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FRESNO (CBS/AP) — Health officials were set to draw blood from hundreds of Yosemite National Park employees as part of a research project that aims to help scientists better understand a potentially deadly virus carried by deer mice.

The hantavirus killed three people and sickened six other visitors to the park this summer.

Officials say more than 300 year-round employees have volunteered to have their blood drawn and answer a questionnaire.

Park epidemiologists and doctors with the California Department of Public Health are trying to determine how many people might have been exposed to hantavirus and why some people get sick and others don’t.

The virus is transmitted on airborne particles of mouse urine and feces, but researchers say little else is known about it. It was first detected in 1993.

No park employees were among those sickened this summer.

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